Laser alignment systems have been used for years, and some of these systems use a rotating laser light source that periodically sweeps 360°, thereby creating a plane of laser light that can be received and detected by a laser light receiver. Other types of laser alignment systems use a transmitter that projects laser energy continuously in a reference cone, or a reference plane, of light that spreads in all directions about a 360° circle. The range of such detection units, especially when used in direct sunlight, is greatly increased by modulating the laser light source at a predetermined frequency. Such a laser light alignment system using a modulated laser light source is disclosed in U.S. Pat. No. 4,756,617.
In this '617 patent, the laser energy in the alignment field is modulated at 8 kHz, and the signals produced by photodetectors of a laser light receiver are filtered and amplified. A split-cell photodetector is used, thereby producing two input signals that each have a particular strength, depending upon the position of the split-cell photodetector as it is being impacted by the modulated laser energy. After these two input signals are run through individual band pass filters and amplifiers having automatic gain control, the ratio of their signal strengths are compared (by a “ratio comparator” circuit 111) to determine the relative position that the laser light is impacting the photodetector receiver.
Unfortunately, there is a common problem when using such laser beam detectors indoors and at short range from the laser light transmitter. In such conditions, it frequently occurs that the laser beam which travels from the transmitter reflects off various surfaces such as walls, windows, or perhaps even light-colored clothing. When this occurs, the laser light detector will respond to the reflected laser energy and perhaps produce an elevation display, even though the laser detector is not directly in the actual laser beam (i.e., the beam that has not been reflected). In a worst case, some laser beam detectors will commonly display an “on-grade” condition whenever the transmitter and laser detector are in the same room, independent of the elevation or orientation of the laser detector.
The cause for such behavior can simply be a matter of sensitivity. Since such laser transmitter and detector systems are designed to operate at relatively large distances, e.g., up to one thousand feet between the transmitter and detector, and since the laser power impinging on the photocells of the detector decreases inversely proportional to this separation distance, most laser detectors are designed with extremely high sensitivity. As a result, when operated with the detector in close proximity to the transmitter, very small amounts of reflected laser power are enough to trigger the laser detector, and often will produce an erroneous elevation signal.
In the past, this problem has been addressed using a variety of methods. One method is simply to reduce the sensitivity of the laser detector. While this reduces the problem of reflected signals, it of course also reduces the operating range of the system, which is typically an undesirable trait.
A second solution for the laser light reflecting problem is to selectively reduce the sensitivity of the laser detector. This can be achieved by having a user control on the laser detector that reduces the sensitivity of the detector unit when operating in close proximity to a laser transmitter. This technique has been used in a “Pocket Level” model laser detector that was produced by Spectra-Physics, and has been described in the following patents: U.S. Pat. No. 4,756,617, U.S. Pat. No. 4,732,471, and U.S. Pat. No. 4,674,870.
A third method of combating the reflection problem is by utilizing a directional film, which is a plastic material that incorporates embedded “microlouvers” to control the optical transmission of the material, based on the incident angle of the incoming light. Such material is produced by 3M Corporation as VIKUITI LIGHT CONTROL FILM™, and is described in the following patents: U.S. Pat. No. 4,764,410, U.S. Pat. No. 4,766,023, and U.S. Pat. No. 5,254,388. The purpose of the film is to limit the angle of light entering the laser detector to only that which is substantially normal to the front surface of the detector unit. In this way, stray laser power from reflections in a small room will largely be rejected and ignored by the laser detector, while the intended direct laser power will be passed through the film and thus detected. Since the “light control” film operates in one axis only, to achieve rejection of straight laser power in both horizontal and vertical axes, then two layers of material must be used having microlouvers arranged on a first layer in the horizontal direction, and the second layer on the vertical direction or axis. This is the method used in the HR200 model laser detector produced by Trimble Navigation, Ltd.
Problems with the above “solutions:” in the second solution described above, user intervention is required. If a user does not realize that the laser transmitter-detector system is in an error-prone situation, or if the user is poorly trained or just forgets, then the user may not activate the short range mode of the detector. In that situation, the user will encounter erroneous elevation indications with no clear method of realizing that this is the case. Similarly, if the short range mode is enabled, and then the range between the transmitter and laser detector increases, then suddenly the system operating range will not be as expected, and the user may not realize that the “short range mode” now having been enabled has become the problem.
With regard to the third solution described above, it should be realized that the “light control film” is comparatively expensive, and moreover, this solution is not complete. If reflections nevertheless occur in a way such that the incident angle of light into the laser detector is mostly normal, then the laser detector unit can still indicate an erroneous elevation data with no indication that there has been a problem due to the reflections. Furthermore, by design the “light control film” limits the acceptance angle of the laser detector. The acceptance angle is the angle at which the detector can be rotated so that it no longer faces the transmitter and will still properly operate. In practice, it is desired that the acceptance angle be as large as possible for ease of operation for the customer/user. This is in direct conflict with the intended operation of the “light control film” and thus use of the film is a detriment for the customer/user.
It would be desirable to provide yet another solution for rejecting reflections of modulated laser light when used with such laser light receivers/detectors. One new way of achieving this is to use a software algorithm that senses a “pattern” of the laser light being received by the photoelements of the detector.